Thursday, May 23, 2013

Brown-headed Cowbird

Once the female Wood Thrush finished building the nest, she didn't stay in the vicinity of the tree as much. She was probably taking the opportunity to put on a little weight before having to sit on the eggs. For a few days, she would be laying one egg a day until her clutch was complete, then she would begin nesting in earnest. 

On an occasion when both the male and female thrushes were away from the nest, a female cowbird flew in and very quickly deposited an egg in the thrush nest. I don't think she was on the nest more than thirty seconds, but I'm pretty sure she laid an egg just that quick. Then she hopped out and flew away just as quickly.

Female cowbirds are brood parasites. In other words, they don't build their own nests, but lay their eggs in the nests of other birds. In fact, cowbird eggs have been found in the nests of over two hundred other host species. A cowbird female can lay up to 36 eggs in a season. Parasitism is the main reason I dislike cowbirds.

Some host females recognize the alien and use different strategies to get rid of the egg. Some birds will build an entirely new nest over the old or even abandon the nest. Some birds will break the egg while others will roll the egg out of the nest. Most, however, don't recognize the foreign egg and raise the chick as it's own once hatched. 

Cowbird eggs hatch a little sooner than most other species, giving the chick an unfair advantage since it is both larger and more mature than the other chicks. The cowbird chick out-competes for the food brought back by the parents. In this photo, you can see the female cowbird leaving the nest which is over on the right.

I couldn't tell if the female thrush recognized what happened while she was gone. She didn't seem to take notice, although I wasn't there to watch the entire time. It may be she rolled the egg out of her nest. I would not have known if she did because we have a ground cover underneath that tree and I would not have been able to see it.

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